1. Field of the Invention
The present invention relates to methods and equipment for dividing a bulk liquid into drops. The term "liquid" is used hereinafter in the broad sense, that is it is intended to mean both liquids and melts of the normally solid materials.
This invention may be used wherever the uniform (monodisperse) drops of liquid are produced in great quantities. The invention is of particular advantage in the case of reducing any molten material to grains, for instance in producing ammonium nitrate, carbamide, sulphur, and compound fertilizers containing nitrogen, phosphorus and potassium.
2. Prior Art
It is common knowledge that granulating materials are effected in granulating towers having a considerable height from 30 to 100 m) and diameter (from 6 to 24 m). The melt is divided into drops in the upper portion of the tower. The melt drops fall down in counter-current to the upward current of the cooling air, are cooled thereby and crystallize to form grains of a spherical shape.
An important problem which is encountered in the practice of granulating materials is the provision of such an apparatus for reducing liquids to drops which being economic in use, simple in construction, relatively not expensive, and highly reliable in operation, would provide for:
(1) producing drops having a predetermined uniform size and a maximum degree of dispersion;
(2) possibility to change in a wide range the output of the apparatus per unit of time without changing the size thereof and affecting the degree of monodispersion of the drops;
(3) such a movement of the drops in the air at the outlet of these apparatus, wherein the drops do not collide and collapse as a result.
Many attempts have been made to solve the above problem, but no satisfactory solution has been proposed so far.
The most efficient among the prior art methods of dividing liquid into drops is a method wherein the liquid to be divided is fed into a perforated container having discharge orifices located at different levels, with the rate of feeding the liquid being controlled depending on the required output, and caused to issue from said orifices in jets with simultaneously maintaining a positive pressure over the free surface of said liquid within said container. The liquid jets thus obtained are divided into drops. (cf. B. G. Holin, "Tsentrobezhnye i vibratsionnye granulyatory rasplavov i raspyliteli zhidkosti", M., Mashinostroenie, 1977, pp. 104-105, ris. 64). According to the method discussed the level of the liquid in the perforated container is maintained constant within the whole range of the feed rate of said liquid so that all the discharge orifices are below the free surface of the liquid and the liquid feed control is effected by varying the pressure over said liquid.
In this case with the change of the liquid feed rate the velocity of the liquid issuing in jets from said container proportionally changes.
An apparatus to carry out the above method comprises a body and a perforated container mounted thereon and provided in its lower portion with a plurality of discharge orifices for outflow of the liquid located at different levels, a compressed gas source, and inlet pipes for feeding said gas and liquid into the cavity of said container. (cf. B. G. Holin, the work referred to above, on the same pages). This apparatus further includes a liquid distributor which communicates with the liquid inlet pipe and has apertures for uniformly introducing the liquid into the perforated container. The constant level of the liquid in this apparatus is provided with the aid of a level controller of a float type.
In spite of apparent advantages that the above method and apparatus have there is a number of difficulties which are encountered when carrying said method into effect with the use of said apparatus.
Thus, in the case of a necessity to considerably change the output of the apparatus (for example, from 2 to 10 times) the rate of feeding the liquid into the perforated container must be proportionally changed, in which case according to this method the gas pressure over the free surface of said liquid within said container has also to be changed, which results in a sharp change of the velocity of the liquid flowing out from said container. As a result the separate jets of the liquid may, in some cases, flow together in the form of a shapeless flow of liquid running over the outer surface of the perforated container.
This results in that the size and weight of the drops are not uniform. Their falling velocity and time in the granulating tower increase or decrease, which in turn disturbs the predetermined conditions of their cooling, thereby affecting the quality of the granules being formed.
Thus, when carrying into effect the prior art process with the aid of the above apparatus the predetermined shape and size of the granules being formed can be obtained only within a narrow range of the output of the apparatus.
To enable controlling the output of the apparatus in a wider range without significant decline in the quality of the granules being formed it becomes necessary to use complex systems including several apparatuses of the above type operating in parallel. A coarse adjustment of the system to provide a required output is effected by operating a selected number of the apparatuses combined in such a system, and a precise adjustment is effected by varying the pressure of gas over the free surface of the liquid simultaneously in all the operating apparatuses (cf. B. G. Holin, the work referred to above, p. 167, FIG. 99).
This, however, complicates to a great extent operating such apparatuses and the granulating tower as a whole, causes higher expenses to carry out granulating operations, and involves more attending personnel.
It should also be noted that this process of dispersion creates conditions for intensive adhering and sticking together of the granules being formed in the lower portion of the granulating tower, which decreases the average output thereof, requires more frequent outages for cleaning, and makes it more hazardous for the attending personnel because of possible crumbling and failure.
Furthermore, the droplets formed with the aid of the prior art apparatus may collide while falling down, thereby affecting their initial shape and weight.
All the above disadvantages are responsible for that the above method and the apparatus are not used on a wide industrial scale.